The present invention relates to an electrophotographic recording material including a dual photoconductive layer containing selenium applied on an electrically conductive substrate.
Electrophotographic recording materials are used for electrophotographic copying processes which have found wide application in the photocopying art. Such processes are based on the property of the photoconductive material to change its electrical resistance when exposed to an activating radiation.
After a photoconductive layer has been electrically charged and exposed to an activating radiation in a pattern corresponding to an optical image, a latent electrical charge image, which corresponds to the optical image, is produced on the photoconductive layer. At the exposed locations, the conductivity of the photoconductive layer is increased to such an extent that the electric charge can flow off, at least in part and through the conductive substrate, but in any event the flow off is at a greater extent at the exposed locations than at the unexposed locations. At the unexposed locations, the electric charge should remain essentially intact, and the pattern of the charge can then be made visible by means of an image powder, a so-called toner. The resulting toner image, if necessary, can then be transferred to paper or a similar record carrier.
Electrophotographically active substances which have been employed include organic as well as inorganic substances. Among the inorganic substances which have been used, selenium, selenium alloys and compounds with selenium have gained particular significance. They play an important role particularly in their amorphous state and have found many uses in practice.
The change in the electrical conductivity of a photoconductor depends on the intensity and the wavelength of the radiation employed. Within the range of visible light which is preferred for practical use in electrophotography, for example in office copiers, the amorphous selenium exhibits high sensitivity on the blue side, i.e. in the short-wave range, whereas on the red side, i.e. in the longwave range, it exhibits a very low sensitivity.
The result is that a red character is reproduced on an electrophotographic plate in the same manner as a black character, which under certain circumstances, particularly with colored masters, may present practical disadvantages, since a black character on a red background--or vice versa--will not be distinguishable from its background and can not, therefore, be made visible. For wavelengths in the infrared range, amorphous selenium is not suitable at all.
In contradistinction to amorphous selenium, crystallized selenium is known to be red sensitive. Thus, the use of crystallized selenium makes possible reproduction involving this part of the visible spectrum. However, the high dark conductivity of crystallized selenium, i.e. its characteristic of being such a good conductor for electric current while in the unexposed state that a charge applied to its surface cannot be maintained for the length of time required for electrophotographic purposes, discourages its use for such purposes.
Additions to selenium, such as, for example, arsenic or tellurium, are known to broaden the spectral sensitivity of selenium into the longer wave spectral range. However, systems comprised of selenium and tellurium alloys present disadvantages in that it is more difficult to prepare the photoconductive layer, since alloys are more difficult to evaporate homogeneously. Moreover, with a higher tellurium concentration, the photoconductive layer exhibits an undesirable tendency toward crystallization and thus has only a short service life.
Moreover, no selenium based photoconductors are known which have a significant and practically usable sensitivity extending into a wavelength range of over 800 nm.
Sensitivity in that range is the desired behavior, however, if electrophotography is also to be used to advantage for other purposes. For example, data output devices operate with infra-red (IR) solid state lasers as radiation sources, in a wavelength range from about 800 to 850 nm. If such radiation is to be detected by a photoconductor, the photoconductor must be sensitive in such a wavelength range.